1. Field of Invention
The present invention relates to the measurement of the wavelength dispersion characteristic of devices under test (DUT) such as fiber pair, and in particular to the measurement of the wavelength dispersion characteristic by connecting separate measuring methodes on both ends of the DUT.
2. Description of the Related Art
In case of light being transmitted over a long distance, the transmission of light only through an optical fiber will involve considerable losses. Therefore, optical fiber transmission lines combined with optical amplifiers (EDFA) for amplifying optical signals are used for the optical fiber to prevent any possible losses. The optical amplifiers let light through only in one direction. Therefore, a bi-directional communication requires a cable integrating an optical fiber transmission line transmitting light in one direction and another optical fiber transmission line transmitting light in the direction opposite to the one direction. This cable is called a fiber pair.
The configuration of a fiber pair is shown in FIG. 6(a). An optical fiber 112 combined with an optical amplifier 114 constitute an optical fiber transmission line 110. The optical fiber transmission line 110 lets light through to the right. An optical fiber 122 combined with an optical amplifier 124 constitutes an optical fiber transmission line 120. The optical fiber-transmission line 120 lets light through to the left. An optical fiber transmission line 110 and a optical fiber transmission line 120 constitutes an optical fiber pair 100a. Incidentally, two sets of fiber pairs are called two fiber pairs as shown in FIG. 6(b). Fiber pairs 100a and 100b constitute two fiber pairs 100.
The configuration of the measurement system for measuring the wavelength characteristic of two fiber pairs is shown in FIG. 7. At one end of a fiber pair 100a, which is one of two fiber pairs 100, a variable wavelength light source 202 is connected and at another end an O/E (optical/electric) converter 302 is connected. At one end of a fiber pair 100b, which is one of the two fiber pairs 100, a fixed wavelength light source 204 is connected, and at another end an O/E (optical/electric) converter 304 is connected. Incidentally, between the variable wavelength light sources 202, 204 and single fiber pairs 100a, 10b, a light modulator may be installed.
To measure wavelength dispersion characteristics, the wavelength λx of the variable wavelength light source 202 is swept, while the wavelength λ0 of the fixed wavelength light source 204 is fixed. The phase difference between the output signals of the O/E converter 302 and the output signals of the O/E converter 304 is measured by the phase comparator 306, to measure the wavelength dispersion characteristic of the two fiber pairs.
Here, in a bulk transmission line constituting a trunk line, two fiber pairs may be secured. In most lines already laid out, often only one fiber pair can be secured. Therefore, it is necessary to measure the wavelength dispersion characteristic of a single fiber pair.